Electrical cables, such as heat trace cables, generally have inner conductive wires that are surrounded by one or more protective layers. The inner conductive wires and the surrounding protective layers are usually made of materials that are flexible enough to bend, but also rigid enough to retain nominal cable dimensions. Occasionally, longer lengths of cable are desired than are normally produced by existing production processes. In order to lengthen the cables, one end of a cable may be joined or appended to an end of another cable. When joining two cables, they must be joined electrically to permit electric current to flow therebetween and mechanically to provide sufficient structure to hold the cables together.
Electrical cables are typically joined electrically by splicing the wires disposed therein together, thereby forming a spliced section or area at the joined ends of the cables. The splicing process may be accomplished by soldering, welding or mechanically clamping the inner conductive wires of the two cables together. However, for such prior art splicing methods the bending strength at the spliced section is less than the bending strength of each of the original cables. Also, for the soldering process in particular, the tensile strength at the spliced section is less since the solder used for the soldering process typically consists of a different material than the wires. Accordingly, there is a need for a splicing process that provides a strong mechanical and electrical connection between the wires of joined cables without substantially sacrificing bending strength or tensile strength at the spliced section.
The use of a sleeve to join the ends of the cables is known in the art. For example, U.S. Pat. No. 4,057,187 to B. H. Cranston, which issued on Nov. 8, 1977, provides a method of joining two wires that includes aligning the ends of the wires within a sleeve and then detonating an explosive composition coated about the exterior surface of the sleeve. However, such explosive splicing processes can become expensive and hazardous to use.
Although the splicing process may provide a certain degree of mechanical support for holding the joined cables together, the present inventors have discovered that substantially greater mechanical support can be provided to the joined cables by encapsulating their spliced section.
Others have used a polymeric shrinking tube in an attempt at providing such mechanical support to a spliced section. In this regard, a spliced section is situated within the polymeric shrinking tube and then the tube is heated to shrink and conform to the outer surface of the spliced section. Examples of such shrinking tube processes may be found in U.S. Pat. No. 4,487,994 to G. Bahder, which issued on Dec. 11, 1984; U.S. Pat. No. 4,822,952 to C. Katz, et al., which issued on Apr. 18, 1989; and U.S. Pat. No. 5,194,692 to D. O. Gallusser, et al., which issued on Mar. 16, 1993.
However, polymeric shrinking tubes present a number of problems due to their lack of strength and flexibility. For example, a polymeric shrinking tube fails to provide the electrical insulation required by third party authorities, such as Underwriters Laboratories (U. L.) or Factory Mutual (FM), of heat trace cables. In addition, polymeric shrinking tubes often fail to prevent liquid, e.g., water/moisture, ingress to the conductive portions of the cables.
U.S. Pat. No. 4,654,474 to L. J. Charlebois, et al., which issued on Mar. 31, 1987, and U.S. Pat. No. 4,678,866 to L. J. Charlebois, which issued on Jul. 7, 1987, each provide a method for joining a pair of cables by providing a grounding bar to structurally bridge the cable ends together. Thereafter, multiple layers of tape are wrapped about the spliced region, including the grounding bar and a polyethylene material is extruded about the spliced region a mold.
U.S. Pat. No. 4,484,022 to H. Eilentropp, which issued on Nov. 20, 1984, provides a method of connecting two cables in which a filler tube is melted and compressed within an enclosed structure in order to produce a bond between the cables and the enclosed structure. The filler tube is made of a copolymer that has a melting or softening point that is considerably below the melting or softening point of the enclosed structure, as well as that of the outer sheath of the two cables.
None of the above patents describe or suggest the use of a polymer having chemical and physical properties that are substantially similar to the polymer layer that adjacently surrounds the wires of the cables, as provided by the present invention.
The present invention overcomes the disadvantages of conventional cable splicing by providing a method for welding the conductors (i.e., metallic wires) followed by polymer encapsulation. The present invention provides an encapsulated spliced section having tensile strength, flexibility, thermal properties, moisture resistance, and dimensional characteristic substantially similar or identical to the polymer sheath which typically encases or insulates the wires. Moreover, the polymer encapsulation section is formed using a powder polymeric material which, under appropriate heating and pressure conditions, forms a polymeric encapsulation section which does not have any voids, i.e., air bubbles, and exhibits substantially similar properties to that of the original flexible polymer sheath of the cables themselves, and physically bonds to the flexible polymer sheath. The present inventors have discovered that if a polymeric material is of a granular form rather than a power form, then undesirable voids can be formed which cause the resultant polymer encapsulation to substantially reduced flexibility, strength, temperature resistance, moisture resistance, and dimensional characteristics.